A man in Florida falls gravely ill and dies under unsettling circumstances. Special Agent Matthew Ryker harbors a suspicion of anthrax, and a chilling question takes shape: how widespread is the infection?

As a new instance of anthrax emerges, Ryker's inquiry shifts its focus to the city of New York. Meanwhile, Bruce Ivins finds himself increasingly troubled by doubts regarding those he works with.

Following the assault on Capitol Hill, Ryker and his team are dispatched to a postal facility now experiencing a surge in anthrax cases.

As the FBI delved deeper into Bruce Ivins's enigmatic history, a former colleague's concerns brought to light unsettling patterns in his behavior. Meanwhile, Ryker and Dani began to unravel the origins of the letters.

A hush fell as attention focused on Mr. Z. Just as the investigation began to narrow in on a potential culprit, Ryker and Dani received new instructions. However, the pursuit of their own information remained undeterred.

The presence of two potential suspects forces Ryker to confront the core of his own convictions regarding the matter.